1. Field of the Invention
The present invention relates to coating compositions based on ethylene-tetrafluoroethylene, (hereinafter referred to as E-TFE) copolymers, which are capable of forming films that adhere perfectly to metal surfaces of different types, even after aging.
2. Prior Art
It is well known that E-TFE copolymers have certain desirable physical and chemical characteristics, such as substantial inertia to chemical reactants in general, good thermal stability and excellent dielectric properties.
In U.S. patent application Ser. No. 142,357, now abandoned, coating compositions based on fluorinated polymeric materials consisting of E-TFE copolymers containing from 50 to 70 mol % of chemically combined tetrafluoroethylene are disclosed.
In fact, it has been found that those coating compositions are capable of yielding films that adhere perfectly to metal surfaces of different types, even after aging.
However, it was also found that the filming properties of those compositions are not quite as good as desired. In fact, in order to obtain complete and reproducible filming of the compositions containing copolymers of such a composition (50-60 mol % of tetrafluoroethylene) as to have a melting point near the maximum value typical of this class of polymeric materials, it is necessary to effect a thermal baking treatment of the coated metal surface at temperatures above 300.degree. C and for baking times often exceeding 2 or 3 minutes.
This requirement for extended thermal baking represents a considerable limitation in the use and a serious disadvantage for such coating compositions. This is especially so when one considers that the required coating conditions greatly exceed the conditions attainable in the standard machines that are commercially used for the continuous coating of metal substrates. Thus, these machines cannot be used with the above described polymeric compositions.